Tracking the positional relationship between a boring tool and one or more buried lines using a composite magnetic signal

ABSTRACT

A boring tool is moved through the ground in a region which includes at least one electrically conductive in-ground line and which is subject to static magnetic fields including the magnetic field of the earth. Tracking a positional relationship between the boring tool and the line, as well as a directional heading of the boring tool within the region are provided by: (i) generating a time varying magnetic field from the line; (ii) at the boring tool, detecting a composite magnetic signal which includes one component affected by the static magnetic fields and another component affected by the time varying magnetic field such that the static magnetic field component varies as a function of the directional heading and the time varying component varies as a function of the positional relationship; and (iii) processing the composite magnetic signal to separate the static magnetic field component and the time varying magnetic field component from the composite magnetic signal for use in determining the directional heading and the positional relationship. In one feature, the static magnetic field component is used to determine the directional heading of the boring tool and the time varying magnetic field component is used to determine the positional relationship.

RELATED APPLICATIONS

The present application is a continuation application of applicationSer. No. 11/074,115 filed Mar. 7, 2005 now U.S. Pat. No. 7,071,695;which is a continuation application of application Ser. No. 10/615,222filed Jul. 7, 2003 and issued as U.S. Pat. No. 6,882,154 on Apr. 19,2005; which is a continuation application of application Ser. No.09/751,478 filed Dec. 28, 2000 and issued as U.S. Pat. No. 6,617,854 onSep. 9, 2003; which is a continuation application of application Ser.No. 09/321,151 filed May 27, 1999 and issued as U.S. Pat. No. 6,191,585on Feb. 20, 2001; which is a continuation-in-part application ofapplication Ser. No. 09/082,142 filed May 20, 1998 and issued as U.S.Pat. No. 5,914,602 on Jun. 22, 1999; which is a continuation applicationof application Ser. No. 08/643,209 filed May 3, 1996 and issued as U.S.Pat. No. 5,757,190 on May 26, 1998, all of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a system including anarrangement for tracking the positional relationship between a boringtool and one or more buried lines such as, for example, pipes, cables,conduits or other conductors and more particularly to an arrangement forindicating certain changes in the positional relationship between theboring tool and the lines based upon a particular characteristic of amagnetic field emanated from the boring tool or a magnetic fieldemanated from each one of the lines. In one aspect of the invention, anarrangement and associated method are provided for tracking thepositional relationship between a boring tool and one or more buriedlines using a composite magnetic signal.

The installation of utility lines underground is increasingly popularfor reasons of aesthetics and for practical reasons such as, forexample, protecting these lines from the effects of severe above groundweather conditions. However, in areas where buried lines have previouslybeen installed, it is undesirable to excavate an entire pathway for thepurpose of installing additional lines since such excavation many timesresults in the unintentional damage of an existing utility line. Areaswhich include buried fiber optic cables are particularly problematic forseveral reasons. First, a fiber optic cable is extremely difficult torepair once it has been severed or damaged. Second, because a fiberoptic cable is capable of simultaneously carrying a vast amount ofinformation, downtime can be quite costly.

In the past, various horizontal boring tool locating and monitoringsystems have been developed which advantageously eliminate the need forexcavating the entire pathway in which a utility line is to beinstalled. One such system is described in U.S. Pat. No. 5,337,002(issued to the inventor of the present invention) which is incorporatedherein by reference. FIG. 1 is taken directly from the '002 patent andillustrates a horizontal boring operation being performed by anapparatus which is generally designated by the reference numeral 10.Boring apparatus 10 includes a drill head 12 incorporating a transmitterwhich transmits a locating signal 14. A portable hand held receiver(locator) 16 is used to detect the locating signal through theintervening earth whereby to ultimately guide the boring tool to aterminating pit 18. The system relies on its operator having priorknowledge of any obstacles in the boring path of the tool such as anobstacle 20 so that the tool can be steered around the obstacle.Unfortunately, such prior knowledge may be inaccurate, if available atall. Moreover, the system of FIG. 1 does not by itself provide a directindication of the relationship between a respective obstacle such as anin-ground line and the boring tool. Without this indication, theoperator is in danger of possibly damaging an in-ground line with noforewarning. In view of the serious consequences of damaging certainburied lines, as described above, operators of boring tools are less andless willing to assume this risk.

As will be seen hereinafter, the present invention provides a highlyadvantageous arrangement and associated method for providing an operatorwith indications which warn the operator when a boring tool isapproaching an in-ground obstacle such as a fiber optic cable. Dependentupon the specific indications provided to the operator, an appropriatecourse of action may thereafter be taken which assures that the boringtool will not damage the line. In another advantage, the presentinvention provides an arrangement which utilizes a composite magneticsignal in determining both the directional heading of the boring tooland the positional relationship between the boring tool and theunderground line(s). The arrangement may be configured so as to enablesimultaneous tracking of the positional relationship between the boringtool and a plurality of in-ground lines whereby to avoid physicalcontact of the boring tool with the lines.

SUMMARY OF THE INVENTION

As will be described in more detail hereinafter, there is disclosedherein a system in which a boring tool is moved through the ground in aregion which includes at least one electrically conductive in-groundline. The system, like the system of FIG. 1, includes a boring tool.However, the system of the present invention includes an arrangement fortracking a specific positional relationship between the boring tool andone or more in-ground lines. The arrangement includes first means forgenerating a magnetic field from one of either the boring tool or theline. Second means for detecting a particular characteristic of thegenerated field is carried by the boring tool if the field is generatedfrom the line or is connected to the line if the field is generated fromthe boring tool. The characteristic varies as a function of the specificpositional relationship between the boring tool and the line as theboring tool moves through the ground within the region. Third means isincluded which is responsive to the detection of the characteristic forindicating certain changes in the positional relationship between theboring tool and each one of the lines being monitored.

In one aspect of the present invention, a boring tool is moved throughthe ground in a region which includes at least one electricallyconductive in-ground line and which is subject to static magnetic fieldsincluding the magnetic field of the earth. Tracking a positionalrelationship between the boring tool and the line, as well asdetermining a directional heading of the boring tool within the regionare provided by: (i) generating a time varying magnetic field from theline; (ii) at the boring tool, detecting a composite magnetic signalwhich includes one component affected by the static magnetic fields andanother component affected by the time varying magnetic field such thatthe static magnetic field component varies as a function of thedirectional heading and the time varying component varies as a functionof the positional relationship; and (iii) processing the compositemagnetic signal to separate the static magnetic field component and thetime varying magnetic field component from the composite magnetic signalfor use in determining the directional heading and the positionalrelationship. In one feature, the static magnetic field component isused to determine the directional heading of the boring tool and thetime varying magnetic field component is used to determine thepositional relationship.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be understood by reference to the followingdetailed description taken in conjunction with the drawings, in which:

FIG. 1 is a cross-sectional view, in elevation, of a prior arthorizontal boring operation which is taken directly from U.S. Pat. No.5,337,002.

FIG. 2 is a diagrammatic plan view of a region in which a horizontalboring operation is being performed by a system manufactured inaccordance with the present invention.

FIG. 3 is a diagrammatic cut-away view of a drill head including amagnetic locating field transmitter manufactured in accordance with thepresent invention.

FIG. 4 is a block diagram illustrating one configuration of a fieldstrength detector of the present invention.

FIG. 5 is a block diagram illustrating one configuration of a telemetryreceiver of the present invention.

FIG. 6 is a diagrammatic illustration of a visual display of indicationsprovided in accordance with one embodiment of the present invention.

FIG. 7 is a diagrammatic elevational view illustrating a drill head on aperpendicular approach path P1 relative to an in-ground line monitoredby the system of the present invention.

FIG. 8 is a diagrammatic plan view illustrating a drill head on a skewedapproach path, P2, relative to an in-ground line monitored by the systemof the present invention.

FIG. 9 illustrates drive signals providable by the locating fieldtransmitter shown in FIG. 3 to its orthogonal antenna elements.

FIG. 10 is a diagrammatic plan view of the region originally shown inFIG. 2 in which a horizontal boring operation is being performed byanother embodiment of a system manufactured in accordance with thepresent invention in which magnetic locating fields are transmitteddirectly from the lines being monitored.

FIG. 11 is a cut-away view of a drill head used with the embodiment ofFIG. 10 illustrating a magnetic locating signal receiver positionedwithin the drill head.

FIG. 12 is a partial block diagram illustrating one configuration of atelemetry receiver for use with the embodiment of FIG. 10.

FIG. 13 is a diagrammatic elevational view illustrating a drill head onan approach path P3 which avoids an in-ground line being approached andwhich is skewed relative to the line. The latter being monitored by thesystem of the present invention.

FIG. 14 is a diagrammatic block diagram illustrating a compositemagnetic signal detection arrangement manufactured in accordance withthe present invention and configured for incorporation into the drillhead of a boring tool.

FIG. 15 is a diagrammatic block diagram illustrating a firstimplementation of a transmission section used in the detectionarrangement of FIG. 14 using an RF carrier for transmission ofinformation to an above ground location.

FIG. 16 is a diagrammatic block diagram illustrating a secondimplementation of another transmission section configured for use in thedetection arrangement of FIG. 14 in cooperation with an isolatedelectrical conductor extending through the drill string.

DETAILED DESCRIPTION OF THE INVENTION

Having described FIG. 1 previously, attention is immediately directed toFIG. 2 which illustrates a plan view of a horizontal boring operationwhich is being performed in a region 30 having first and secondin-ground lines 32 and 34, respectively. It is to be understood that twolines are shown for illustrative purposes only and that the presentinvention may be configured for use in regions which include any numberof buried lines. First line 32 is partially accessible in a first pit36, while second line 34 is partially accessible in a second pit 38facilitating connections to either one of the lines. For purposes of thepresent invention, lines locatable within region 30 must include anelectrically conductive component which extends along the length of theline. Even in fiber optic cables, an electrically conductive componentis normally provided in the form of a shield or an electricallyconductive locating lead. Such an electrically conductive component isuseful for either emanating or for receiving a time varying magneticfield in accordance with Maxwell's equations. In the present example,line 32 includes a conductor 40 while line 34 includes a conductor 42.Although the present example shows direct connection to the lines, thetransmitted or receiving signal on or from the line could be inducedwithout direct connection.

Referring now to FIG. 3 in conjunction with FIG. 2, the horizontalboring operation is performed using a system manufactured in accordancewith the present invention and generally designated by reference numeral50. System 50 includes a boring tool 52 having a drill head 54 which isguided on a drill string 56, the latter of which extends from drill head54 to an operator station 58 located in a pit 60 along with an operator62. The process can also be conducted from the surface without the needfor pit 60. It should be understood that when the claims refer to aboring tool, the drill head of the boring tool is specificallyreferenced. Boring tool 52 is typically of the guided type which is wellknown in the art and is configured to transmit information from atransmitter 64 as a signal 66 from various sensors (not shown) which arepositioned in the drill head for detecting the orientation of the drillhead including, for example, its pitch, roll and even possibly yaw. Theconfiguration of these sensors, transmitter 64 and the specificconstruction of the drill head casing, which permits the emanation ofthe telemetry signal through the casing, are described in detail in thepreviously referenced '002 patent. Therefore, these descriptions willnot be repeated herein for purposes of brevity.

A hand-held telemetry receiver or locator for providing indicationsbased upon telemetry signal 66 is also described in the '002 reference.The present system does not require this locator, although it could beused as a means for relaying signal 66 to a telemetry receiver 68 or thesignal could be transmitted directly to receiver 68 positioned atoperator station 58. In this way, telemetry signal 66 providesindications regarding the orientation of the drill head directly (orindirectly) to operator 62 of system 50. It should be understood thattelemetry receiver 68 may be provided integrally with boring tool 52 orin other embodiments may be provided in its own separate enclosure (notshown) which is positionable at operator station 58 by providingsuitable means. Moreover, telemetry receiver 68 could form part of thehandheld locator for processing signal 66 at the handheld locator.However, for purposes of simplicity, it will be assumed that thereceiver is at the operator station.

Continuing to refer to FIGS. 2 and 3 and in accordance with the presentinvention, system 50 includes an arrangement which is generallyindicated by the reference numeral 70 for tracking the specificpositional relationships between drill head 54 and first and secondlines, 32 and 34, respectively. Arrangement 70 includes a magneticlocating signal transmitter 72 which is positioned in drill head 54.Magnetic locating signal transmitter 72, in the present embodiment,includes a field transmission/control unit 74 and an antenna array 76for transmitting a magnetic locating field 78. Antenna array 76 includesthree separate, but essentially identical, antennas 76 a, 76 b and 76 cwhich are arranged orthogonally with respect to one another and whichmay be, for example, dipole antennas. A plurality of electricallyconductive leads 80 electrically interconnect the antenna array withfield transmission/control unit 74 such that each antenna isindividually connected with the transmission/control unit. Therefore,transmission of different signals from the individual elements whichmake up the antenna array is facilitated by simply driving each elementwith an appropriate drive signal. When transmission/control unit 74 isactive, current flows through conductive leads 80 and antenna array 76.This current flow results in the emanation of magnetic locating field 78from the antenna array along orthogonally opposed x, y and z axes, asindicated in FIG. 3, with the z axis being perpendicular to the plane ofthe figure. Magnetic field 78 includes an x axis component 78 a emittedby antenna 76 a, a y axis component 78 b emitted by antenna 76 b and a zaxis component 78 c emitted by antenna 76 c. It is to be understood thateach axis component is actually a three dimensional field. Thetransmission/control unit is configured to include circuitry which isdependent upon the specific way in which it is desired to transmitmagnetic locating field 78. Power, which is presently available at thedrill head, for the operation of telemetry transmitter 64 is alsosuitable for the operation of transmission/control unit 74. However, itshould be appreciated that any well regulated power source may beutilized. It is to be understood that the circuitry for producing theantenna drive signals to be described immediately hereinafter and forreceiving the magnetic fields generated thereby at the telemetryreceiver may be readily provided by one having ordinary skill in the artand may be embodied in many different forms within the scope of thepresent invention. Field 78 is separate and distinct from telemetrysignal 66 by any suitable means, such as frequency differentiation ortime modulation.

Referring now solely to FIG. 2, magnetic locating field 78 is incidentupon lines 32 and 34 within region 30. Arrangement 70 includes fieldstrength detectors 82 and 84 which are in communication with therespective conductors of lines 32 and 34. Several different techniquesmay be utilized to couple the signal induced within each conductor bythe magnetic field to its respective field strength detector. Forexample, detector 82 is inductively coupled with conductor 40 using aninductive clamp arrangement 86 of the type which is well known in theart. Detector 84, on the other hand, is coupled with conductor 42 ofline 34 using a simple electrically conductive lead 88.

Turning now to FIG. 4, each of detectors 82 and 84 are essentiallyidentical internally and incorporate circuitry which is readilyprovidable by one having ordinary skill in the art. For purposes ofsimplicity, only detector 84 is depicted in block diagram form. Thesignal induced by field 78 in conductor 42 of line 34 is transferred todetector 84 via inductive clamp arrangement 86. Thereafter, an inputcircuitry block 90 processes the induced signal as needed using, forexample, amplification and filtering circuitry (neither of which isshown). This processed signal is then transferred to a transmitter block92. Typically, the processed signal is applied to a carrier (not shown)by, for example, frequency modulation and transmitted from an antenna 94as a detection signal 96. Signals (not shown) from other detectors inthe system such as, for example, detector 82 are also modulated ontoindividual carriers.

Turning now to FIGS. 2 and 5, signal 96 from detector 84 along withsignals from other detectors forming part of the system are received bytelemetry receiver 68 at operator station 58. Telemetry station 68includes circuitry which is readily providable by one having ordinaryskill in the art and is configured to receive the detector signals in away which distinguishes the signals from one another. In the presentexample, the telemetry receiver includes channel A for detector 82 andchannel B for detector 84. Similar channels may be provided for anyother lines being monitored. Another method of communication is to timeshare one channel, but this discussion will be limited to using twochannels for purposes of clarity. Since each channel incorporatesessentially identical functional blocks, the present descriptions willbe limited to channel B. The frequency modulated signal 96 transmittedby detector 84 is received by an antenna 98 and then passed by asufficiently narrow band pass filter 100 having a center frequency whichis centered on the carrier frequency of signal 96. The signal is thenpassed to a demodulator 102 whereby to recover the original detectedsignal. Thereafter, the detected signal is passed to an analog todigital converter (A/D) 104, whereby to convert the signal into anappropriate digital format. Following conversion by the A/D, the digitalinformation is transferred to a Central Processing Unit (CPU) 106 forprocessing. Telemetry receiver 68 may incorporate a number of otherfunctional blocks, if desired. For example, a mixer (not shown) can beused to heterodyne the modulated carrier to a lower frequency.

Continuing to refer to FIGS. 2 and 5, CPU 106 also receives data from adrill string monitoring unit 108. The latter provides data to CPU 106including parameters such as the overall length L1 of drill string 56 aswell as the current rate at which the drill string is being fed out.Further description of the internal configuration of drill stringmonitoring unit 108 will not be provided herein since methods andapparatus for determining the parameters of interest are readilyprovided by one having ordinary skill in the art. Alternatively, theactual length L1 of the drill string could also be monitored visually bythe operator and entered into the CPU manually.

Using the above-described input data, CPU 106 performs an analysis ofchanges in the magnitude of the intensity of the locating field in apredetermined way, which will be described hereinafter, and relatesthese intensity changes with movement of the drill head whereby togenerate indications as to the positional relationship of each monitoredline with the drill head. Specific aspects of this analysis will bedescribed at appropriate points hereinafter. In the present example, CPU106 is configured to generate indications of the positionalrelationships in the form of predetermined audio signals. These audiosignals are transferred to an audio amplifier 110 for amplification. Theamplified audio signals are then presented to the system operator via apair of headphones 112 and/or via a remote speaker (not shown). Itshould be mentioned that signals received from any number of detectorsmay be processed by the CPU as provided by each separate channel withinthe telemetry receiver and that the CPU is itself specificallyconfigured to separately track the positional relationship for each linewith the drill head. In this regard, receiver 68 would include a readilyprovidable multiplexer (not shown) positioned at the input of thereceiver for multiplexing the different input signals to the variouschannels or positioned to receive outputs from the CPU to, for example,multiplex the audio signals to the operator, which audio signals couldhave different tones.

Referring to FIG. 6, in other embodiments, indications may be providedto the system operator in forms other than or in combination with audiosignals. For example, telemetry receiver 68 may incorporate a flat paneldisplay 113 which provides a visual indication as to the variouspositional relationships between the monitored lines and the drill head.All of these configurations are considered to be within the scope of thepresent invention provided that the system operator remains apprised ofthe indications being provided. It should also be mentioned that thearrangement of the field strength detectors in combination with thetelemetry receiver may be configured in many other ways, as comparedwith the arrangement of FIG. 2. For example, in one alternativeembodiment, the detectors may be hard-wired to the telemetry receiver,avoiding the need for a wireless link therebetween. In still anotheralternative embodiment, particularly in cases where the detectorconnections to the monitored lines are made nearby the boring tool, thedetector circuitry may be incorporated directly into telemetry receiver68. In yet a further alternative, the detector signal could betransmitted to the receiver via the locator, as indicated previously.

The discussion above generally describes the components which make up asystem in accordance with one embodiment of the present invention. Asdescribed, the system provides indications as to the positionalrelationship of the drill head with the lines being monitored by thesystem. In order to provide a high degree of accuracy in indicationsissued by the system, the present invention advantageously utilizes avector sum derived from preselected components of the magnetic locatingfield generated by the system and present within region 30. Thepreselected components of the field which are monitored may include anycombination of one or more components of the field oriented in spacealong three orthogonally disposed axes dependent upon the particularmonitoring application encountered. Of course, in an application whereonly one component is monitored, the magnitude of the vector sum simplycomprises the magnitude of that particular component.

Generally, the vector sum concept used herein is most useful withmagnetic locating fields which exhibit monotonic variations in themagnitude of their vector sum with distance. In other words, as thesource of the overall field is approached, the intensity of the fielddisplays a continuous increase. A locating field possessing thischaracteristic yields unambiguous distance relationship indicationsthroughout region 30. At this juncture, it is sufficient to say that thecharacteristics of the locating field and the way in which the vectorsum is derived are important considerations which should not beoverlooked in configuring the system for a particular application.

The magnetic locating field can be generated from the drill head anddetected at the monitored lines, as in the embodiment describedimmediately above, or the field can be generated from the monitoredlines in region 30 and detected at the drill head, as will be seen belowin one preferred embodiment. Magnetic locating fields can be generatedand detected in virtually an unlimited number of ways within the scopeof the present invention. It is to be understood that the monitoringoptions provided by the various embodiments of the invention disclosedherein are intended to encompass the greatest possible range oforientations between the drill head and the monitored lines. Theseoptions provide an operator with a degree of flexibility and selectiveaccuracy which has not been seen heretofore. Therefore, the specificembodiment which is useful in a particular orientation could bedetermined in view of, for example, any advance knowledge of theorientation of the boring tool with the cables being monitored.

Reference will now be taken to two examples which follow immediatelyhereinafter illustrating two different approach paths of the drill headto a monitored line. These paths will be described as being monitored bythe embodiment of FIG. 2 in which the magnetic locating field or signalis transmitted from the drill head such that the field will ultimatelyprovide a vector sum which increases in a monotonic fashion as amonitored line is approached.

Referring now to FIG. 7 in conjunction with FIGS. 2 and 3 and inaccordance with a first approach path example, drill head 54 isillustrated on a collision course with line 34. It is also noted that inthis example, line 34 is oriented perpendicular to drill string 56. Inthis orientation, the magnetic locating field may be transmitted fromthe drill using only antenna 76 b which transmits the y axis componentof locating field 78. This y axis component will provide for a trueindication of the distance to line 34. Three positions of drill head 54successively approaching line 34 along a path P1 (shown by an arrow) areindicated as A, B and C. Positions A, B and C are at distances d1, d2and d3, respectively, from line 34 wherein d2 is equal to one-half of d1and d3 is equal to one-half of d2. As the drill head approaches line 34,the induced signal strength can be shown to increase inversely with thedistance of the drill head from the line. At a certain predeterminedmagnetic field intensity, corresponding to a given distance, an initialindication is provided by CPU 106 that the drill head is approachingline 34. In this particular example, the initial indication is providedat distance d1. At this distance, the operator of the boring tool knowsthat the drill head is at a safe distance from the line. However,further indications may warn the operator of an impending collision orat least of the risk of a collision. These indications are providedbased upon predetermined relative increases in the magnetic fieldintensity. While indications herein are based on a relative halving ofthe distance, other relative factors (25% decrease, etc.) may beeffectively used.

Continuing with the first approach path example, should the operatorproceed, following the initial indication at position A, the drill headwill reach position B. As the boring operation continues, CPU 106continuously correlates data from drill string monitoring unit 108,regarding forward movement of the drill head, with the magnetic fieldintensity data. Correlation of these data is useful in certainorientations for estimating the distance to the monitoring line and forpredicting a collision with the line. At position B, the detectedmagnetic field strength will be greater than its strength at position A,since the distance between the drill head and the line is now d2, whichis less than d1. At d2, CPU 106 will issue a second indication to theoperator which advises that the drill head has reduced its distance tothe line by a predetermined fraction since the initial warning wasprovided. Should the operator once again decide to proceed, the drillhead will reach position C. At position C, a third indication is issued,once again advising the operator that the drill head has reduced itsdistance to the line by the same predetermined fraction since the lastindication and, therefore, that the strength of the magnetic field hasincreased by the same predetermined relationship. If the operatorproceeds, indications will continue to be provided each time thedetected magnetic field strength doubles. If the drill head is headingtoward the line at an essentially uniform rate of speed, it should nowbe evident that these indications will occur at a rapidly increasingrate corresponding with a geometric progression. For example, if thepredetermined magnetic field increase corresponds to halving thedistance and the initial indication is provided at time t₁, a secondindication is provided at time t₂ and a third indication is provided ata time t₃ which is Δt/2 after t₂ wherein Δt is equal to t₂−t₁. A fourthindication will then be provided at time t₄ which is Δt/4 after t₃, etc.

In the perpendicular approach path example of FIG. 7, the correlation ofdata described above regarding the feed rate or actual feed distance ofthe drill string in conjunction with the detected rate at which themagnetic field intensity is increasing will specifically indicate thatthe detected intensity of the magnetic field is increasing at the mostrapid possible rate. Under these specific circumstances, CPU 106 may beconfigured to indicate to the operator that the drill head is on acollision course with the line by, for example, a sustained audio toneor by a visual warning such as the one illustrated for Line 1 in FIG. 6which includes an estimated distance to impact or collision. Of course,the operator should stop the boring operation upon receipt of such anindication.

Turning now to FIG. 8, which diagrammatically illustrates a secondapproach path example, a line 114 is monitored by the system of thepresent invention. Drill head 54 is approaching line 114 on a collisioncourse approach path P2 (indicated by an arrow) which is skewed to thepath of the line. As the head approaches, it also rotates about itslongitudinal (x) axis. For reasons which will become evidenthereinafter, drill head 54 is shown approaching line 114 at successivepositions D, E and F which are at distances d1, d2 and d3 (identicalwith those distances shown in FIG. 7) from line 114. As described above,drill head 54 is configured with antenna array 76 which transmitsmagnetic locating signal 78 from the drill head along three orthogonallydisposed axes. It is noted that in this approach path, care must betaken in selecting an appropriate locating field transmission scheme inorder to arrive at a properly derived value for the vector sum which isnot subject to ambiguous readings resulting from peaks or troughs whichmay be present in the magnetic locating field. To that end, the signalfrom which the vector sum is derived for approach path P2 will beinduced in line 34 by the y and z components (see FIG. 3) of signal 78which are emitted by antenna elements 76 b and 76 c, respectively. Thereis no need to transmit a locating field having an x axis component if,for example, the system operator knows that this is the particularoperational situation to hand. In fact, transmission of x axis componentcould result in peaks and/or troughs in field intensity, if it issimultaneously transmitted with other components. It should be notedthat a locating field which is simultaneously transmitted at the samephase and frequency from antennas 76 b and 76 c will contain peaks andtroughs such that the overall locating field does not necessarilyexhibit a monotonically increasing intensity as the drill headapproaches, resulting in possible ambiguous indications as to thepositional relationship.

In order to avoid this possible ambiguity, the locating field isgenerated alternately at predetermined intervals from antennas 76 b and76 c, respectively. At a signal strength detector (not shown) connectedwith line 114, a time average is taken based upon the signal induced byeach component and the magnitude of the vector sum is calculated by CPU106. Circuitry for arriving at this particular vector sum is readilyprovidable by one having ordinary skill in the art and is therefore notillustrated.

Use of the vector sum is advantageous in one aspect in that, at distanced1, the magnitude of the vectoral sum of the magnetic locating signalcomponents induced in line 114 is identical at this distanceirrespective of the specific drill head approach path relative to theline. In other words, the magnitude of the induced signal will beidentical at position A of FIG. 7 and at position D of FIG. 8 because ineach case the drill head is at distance d1 from lines 34 and 114,respectively. Also, the magnitude of the induced signal will beidentical at distance d2 and once again at distance d3. Therefore, CPU106 issues an initial indication to the operator based on the fieldstrength at position D and, thereafter, at points where the relativeintensity of the field has doubled, as in the previous example. Thesesubsequent indications will be provided at position E and then again atposition F corresponding with distances d2 and d3. Unlike the previousexample, however, analysis by CPU 106 of the drill string feed rateprovided by drill string monitoring unit 108 in view of the detectedrate at which the field strength is increasing and/or total feed lengthL1 will result in a determination that a collision will occur aftertraveling a greater distance along path P2 than the distance along pathP1 at the same signal strength. That is, the system provides a pathlength distance to line 34 rather than a perpendicular distance from theline.

Referring once again to FIG. 2 and returning to a discussion regardingthe monitoring of a plurality of lines within region 30, because CPU 106is configured to individually track the positional relationship for eachline being monitored, indications provided to the operator may alsoreadily indicate the specific line for which the indications areapplicable. For example, in the embodiment of the invention illustratedin FIG. 2, audio indications which are provided to operator 62applicable to line 32 may be prefaced by a single, distinct beep or tonesustained for a predetermined interval. Indications which are applicableto line 34 may be prefaced, for example, by a double beep or separatetone. In this way, indications for any number of monitored lines may bedistinctly recognized. Visual formats may, for example, displayinformation regarding the status of each monitored line. FIG. 6, showsthe status for lines 34 and 32 (using like reference numbers forpurposes of simplicity) including a collision warning for line 34. Oneskilled in the art will appreciate that the operator of the system maybe apprised in an unlimited number of ways as to which line specificindications pertain, using different visual formats and differentfrequency tones. These variations are all considered to be within thescope of the claimed invention.

As previously mentioned, a magnetic locating field may be transmittedfrom the drill head and detected at the monitored lines in a number ofdifferent ways. The intent of these transmission options is to reduce oreliminate troughs and peaks in the magnetic locating signal which maycause ambiguous indications. It is to be understood that the circuitryfor producing the antenna drive signals to be described immediatelyhereinafter and for receiving the magnetic fields generated thereby atthe telemetry receiver may be readily provided by one having ordinaryskill in the art and may be embodied in many different forms within thescope of the present invention.

FIG. 9 illustrates antenna drive signals produced by one embodiment oftransmitter 72 in which two elements at a time of the antenna array aredriven by the same frequency sinusoidal waveforms 120 and 122 which arephase shifted by 90° with respect to one another. Transmission isalternately switched between three successive pairs, 76 a/76 b, 76 b/76c and 76 a/76 c, of the three element antenna array such that theoverall locating field is transmitted from three orthogonally disposedaxes. As an example, the antenna element pairs may be driven at afrequency of 80 kHz and the pairs are switched at a 1 kHz rate. A simplelow pass filter (not shown) having a roll-off frequency at approximately100 Hz may be incorporated in telemetry receiver 68 for removing thecomponents of the 1 kHz switching frequency and providing an outputsignal which is directly proportional to the average detected strengthof the detected magnetic locating field. In the example shown in FIG. 8,driving the y and z axis antennas with the drive signals contemplated byFIG. 9 would eliminate the effects of rotation about the x axis andprovide the desired monotonically increasing field strength at line 34.

Another embodiment of transmitter 72 is configured to simultaneouslydrive the three antennas at slightly different frequencies usingsinusoidal waveforms (not shown). For example, antenna 76 b might bedriven at 80 kHz while antenna 76 a is driven at 100 Hz below 80 kHz andantenna 76 c is driven at 100 Hz above 80 kHz. The received signal isthen averaged over a period which is long compared with 100 Hz.

Many modifications may readily be performed in accordance with theteachings of the present invention by one having ordinary skill in theart in order to embody still other forms of the present invention. Forexample, in one highly advantageous modification, the locating signalmay be transmitted by the monitored lines, as will be describedimmediately hereinafter.

Attention is now immediately directed to FIG. 10, which once againillustrates area 30 in which a horizontal boring operation is beingperformed using another embodiment of a system manufactured inaccordance with the present invention and generally designated byreference numeral 200. System 200 includes previously described boringtool 52 having drill head 54 positioned on drill string 56. Telemetrysignal 66 is transmitted from a transmitter 64 (shown in FIG. 11) withinthe drill head. In the interest of brevity, descriptions of other likecomponents will not be repeated.

In accordance with the present invention, system 200 includes anarrangement which is generally indicated by reference numeral 220 fortracking the specific positional relationships between drill head 54 andfirst and second lines, 32 and 34, respectively. Arrangement 220includes a first field signal transmitter 222 in electricalcommunication with conductor 40 of first line 32 via an electricallyconductive lead 224 and a second field signal transmitter 226 inelectrical communication with conductor 42 of second line 34 via anelectrically conductive lead 228 or inductive clamp. In this preferredembodiment, each of the field signal transmitters is essentiallycomprised of a readily providable oscillator which oscillates at apredetermined fixed frequency whereby to drive the electricallyconductive component of its associated line at this fixed frequency.Although we will describe the system as using two frequencies f1 and f2,a single time shared frequency could be employed. First line transmitter222 oscillates at a frequency f1 while second line transmitter 226oscillates at a frequency f2. A first magnetic locating field 230 isemanated from line 32 at frequency f1 while a second magnetic locatingfield 232 is emanated from cable 34 at frequency f2. It should be notedthat locating fields 230 and 232 exhibit a desired monotonic increase inintensity with no troughs or peaks present in the field as the lines areapproached.

Continuing to refer to FIG. 10 and referring to FIG. 11, arrangement 220includes a locating signal receiver 234 positioned within drill head 54.Locating signal receiver 234 includes an antenna array 236 comprised ofthree orthogonally opposed antennas 236 a, 236 b and 236 c which may be,for example, dipole antennas. A plurality of electrically conductiveleads 238 electrically interconnect the antenna array with a locatingsignal receiver/discriminator 240. Although, antennas 236 are shown asbeing individually connected with receiver/discriminator 240, they mayalternatively be connected in series with one another (not shown) to thereceiver/discriminator. Receiver/discriminator 240 cooperates with theorthogonal arrangement of antenna array 236 whereby to detect theintensity of each of magnetic fields 230 and 232 as a vector sum. A lead242 electrically connects receiver/discriminator 240 with previouslydescribed transmitter 64 for transmitting coded magnetic field strengthdata from the transmitter via telemetry signal 66.

Referring now to FIG. 12 in conjunction with FIGS. 10 and 11, theillustrated portion of a telemetry receiver 243 shows a receiver 244which receives telemetry signal 66 including the magnetic field data.Receiver 244 initially receives signal 66 via an antenna 245.Thereafter, the receiver outputs the detected magnetic field strengthdata on a line 246 to a CPU 248. It should be appreciated thatreceiver/discriminator 240 may provide detected field strength data tothe cooperating CPU via the intervening wireless link in an unlimitednumber of ways. For example, receiver/discriminator 240 may digitallyencode data (not shown), which indicates the vectoral sum detectedduring a particular interval for one of the monitored lines. The datafor the respective lines may then be multiplexed using readilyprovidable circuitry and then transferred to transmitter 64 via lead242. Other data derived from signal 66 is output on a line 250 toappropriate sections of the telemetry receiver which are devoted tofunctions such as, for example, analyzing drill head sensor informationor depth.

In accordance with the present invention, CPU 248 is configured toanalyze the magnetic field strength data in conjunction with data fromdrill string monitoring unit 108 so as to provide indications in amanner similar to that described with regard to previously described CPU106 in FIG. 5. These indications are provided to operator 62 via anaudio amplifier 250 and headphones 252 or in a visual format as shownpreviously, for example, in FIG. 6. System 200 provides highly accurateindications as to a broad range of positional relationships includingthose previously described. Accurate indications are also provided forthe still further complex positional relationship shown partially byFIG. 10, as will be described in detail immediately hereinafter.

Referring to FIG. 13 in combination with FIGS. 10 and 11, drill head 54is diagrammatically illustrated proceeding on an approach path P3(indicated by an arrow) toward line 34 which is skewed relative to theline and which represents a course which would ultimately avoid acollision with the line. The drill head is shown at positions G, H and Jnearing line 34. Like the previous examples, the illustrated drill headpositions are at distances d1, d2 and d3, respectively, from the line.For purposes of simplicity, the embodiment of system 200 is configuredin a manner similar to the previously described embodiment whichprovides an initial indication at a locating field intensity resultingin an overall vector sum magnitude corresponding to distance d1 atposition G. Thereafter, indications are provided at points d2 and d3corresponding with positions H and J, respectively, at which themagnetic field magnitude doubles relatively. It is noted, that the useof relative doubling in intensity is arbitrary. However, in thisexample, d3 at position J represents the nearest approach of the drillhead to the line. Assuming that, for example, the system operator iswilling to receive four indications prior to terminating the illustratedboring operation, the indication at position J represents only the thirdindication received by the operator. Upon proceeding, the operatoranticipates a fourth indication, however, no further indications arereceived since the drill head safely passes under the line at position Jwhereupon the detected vector sum intensity begins to decrease. In theevent that a fourth indication were received by the operator, the boringoperation should cease and steps should be taken to ensure that acollision is avoided. In this particular scenario, the system of thepresent invention has advantageously provided indications to the systemoperator that the drill head is on a safe approach path relative to line34 whereby to permit expedient completion of the boring operation. CPU248 in cooperation with drill string monitoring unit 108 could predictthat there would not be an impact based on the distance moved versusrelated data as to the strength of the magnetic field.

Receiver/discriminator 240 and field signal transmitters 222 and 224 maybe configured in an unlimited number of ways in accordance with thepresent invention. For example, the fields can be transmitted atdifferent frequencies f1 and f2, as described, from the field signaltransmitters. In this case, the fields may be continuously transmittedand the receiver discriminator includes circuitry which is readilyprovidable such as, for example, band pass filters having centerfrequencies at f1 and f2 for distinguishing the signals from oneanother. Alternatively, the field signal transmitters may becooperatively configured so as to alternately transmit their respectivemagnetic locating fields at predetermined intervals.Receiver/discriminator 240 then alternately detects each respectivefield. In fact, the use of this scheme permits the fields to betransmitted at the same frequency with f1 equal to f2. It is noted thatcircuitry for implementing this configuration is also readily providableby one having ordinary skill in the art.

Regardless of the way in which the receiver/discriminator cooperateswith the field signal transmitters, field strength data for each line isthen transferred to previously described transmitter 64 via 242 forsubsequent transmission to telemetry receiver 243 as part of signal 66which also contains data derived from orientation sensors (not shown)present in the drill head. Telemetry receiver 243 is positioned atoperator station 58 or may be incorporated into a locator (such aslocator 16 shown in FIG. 1). In another modification, a locator (notshown) can receive one telemetry signal from the drill head and transmitanother telemetry signal back to receiver 243. In this manner, thelocator acts like a repeater station with or without intermediateprocessing.

The use of the vector sum concept in this preferred embodiment is highlyadvantageous in that it provides a high degree of accuracy in theindications given to the operator of the system upon approaching aparticular line, irrespective of the approach path or the orientation ofthe drill head with respect to the approached line. It should beappreciated that path P3 in FIG. 13 represents a complex, but common,approach path.

Like previously described embodiments, the embodiment described withregard to FIGS. 10 through 13 may be modified in any desired way asrequired by a particular application. For example, the vector sum may bedetermined by using one or two antennas, rather than three. However, onehaving ordinary skill in the art will appreciate that such modificationswill not normally be necessary when this embodiment is used. While thisembodiment is accompanied by a number of advantages, one particularadvantage resides in the fact that the drill head often rotates as itadvances. Such rotation does not affect positional relationshipindications in this embodiment since the vector sum is indicative of thefield strength irrespective of the orientation of the drill head withthe line.

In view of the preceding examples, the system of the present inventionis seen to be highly advantageous over prior art techniques andapparatus in each approach path described. While only three approachpaths have been described, it will be appreciated that the describedpaths are applicable to essentially any operational situationencountered by the system.

Attention is now directed to FIGS. 10 and 14 regarding a highlyadvantageous arrangement and associated method of tracking thepositional relationship between drill head 54 and one or more buriedunderground lines using a composite magnetic signal detectionarrangement generally indicated by the reference number 300 in FIG. 14.Detection arrangement 300 includes a magnetometer section 302 within adashed line and an accelerometer section 304 within another dashed line.Magnetometer section 302 is highly advantageous in being configured todetect not only time varying locating fields such as magnetic locatingfields 230 and 232 emitted from buried cables 32 and 34, respectively,but is also configured for detecting static magnetic fields includingthe earth's magnetic field. Details relating to this configuration willbe discussed below following a description of other components includedin detection arrangement 300.

Referring to FIG. 14, magnetometer section 302 includes a magnetometerarrangement configured for detecting magnetic fields (both static andtime varying) along three orthogonal axes. In the present example, theuse of giant magnetoresistive sensors (hereinafter GMR) is contemplatedfor this purposes, however, other suitable devices may be used. Theorthogonally arranged GMR's in magnetometer section 302 are indicated asGMR_(x), GMR_(y) and GMR_(z) using typical orthogonal axis nomenclature.Presently, the GMR is thought to be advantageous in the presentapplication at least in part to the relatively small size of the device.For example, the dimensions of a typical GMR may be 0.2 inches by 0.3inches. The GMR has been used, for example, on computer disk readingheads and as a compass sensor. GMR's typically have a frequency responsethat ranges from static to several mega Hertz. Irrespective of theparticular form of the magnetic detector that is used, it should beappreciated that a composite magnetic signal is detected. That is, thesignal, in this instance, will include both a static and a time varyingcomponent. The three GMR's are interconnected to a multiplexer 306 whichsends multiplexed composite magnetic signal information to an analog todigital (A/D) converter 308. A converted digital signal produced fromthe composite magnetic signal by A/D 308 is then fed into a digitalsignal processor (DSP) 310. At the same time, accelerometer section 304includes a triaxial accelerometer made up of three orthogonally arrangedaccelerometers indicated as A_(x), A_(y) and A_(z). One device suitablefor use as an accelerometer in the present application is the AnalogDevices ADXL05. Like the GMR signals, three accelerometer signals aremultiplexed using a multiplexer 312 and then digitally converted usingan analog to digital converter 314. While the depicted configurationuses separate multiplexers and A/D converters for the magnetic andaccelerometer signals, it should be appreciated that a singlemultiplexer and A/D may just as readily be used for both sets ofsignals.

Still referring to FIG. 14, in accordance with the method of the presentinvention, DSP 310 transforms the composite magnetic signal data toaccount for the instantaneous orientation of the drill head in view ofdata obtained from accelerometers A_(x), A_(y) and A_(z). Thereafter,DSP 310 filters the transformed magnetic data to obtain both a staticcomponent and a time varying component at the induced signal frequencyon the buried conductors (FIG. 10). The static and time varyingcomponents are then passed to a CPU 316 which uses the static componentto determine the heading of the drill head and the dynamic component todetermine proximity of the drill head to the conductor. It should beappreciated that the foregoing operations may all be performed withinthe drill head. Alternatively, the magnetic and accelerometerinformation may be transferred to an above ground location at almost anypoint in the block diagram of FIG. 14 for processing above ground. Inthe present example, information is passed to an above ground locationusing a transmission section 318 to be described at appropriate pointsbelow. The acceleration data may be presented above ground in the formof roll and pitch, as described in U.S. Pat. No. 5,767,678, which iscommonly assigned with the present application, or as individualaccelerations. The magnetic data may be presented in terms of componentsor as a north/south heading. The manner of presentation above ground maybe determined by convenience to the operator or by secondary processingrequirements such as, for example, automated steering of the drillingtool.

The present invention represents a significant advance in boring systemtechnology through the use of a composite magnetic measurement fromwhich static and dynamic components are separated. In the prior art,systems have been produced which incorporate accelerometers and/ormagnetometers in a drill head. As examples, see U.S. Pat. Nos. 3,862,499issued to Isham et al. and 4,875,014 issued to Roberts et al. However,the contemplated application in prior art systems of which the applicantis aware was limited to determining orientation parameters (i.e., pitch,roll and yaw or heading) of the drill head. These parameters may beintegrated to estimate the path of the drill head. The presentapplication, in contrast, is considered to provide sweeping advantagesover these prior art configurations and methods by providing fordetermination of orientation parameters in conjunction with highlyeffective obstacle proximity detection using composite magnetic signaldetection and processing. In this regard, all of the advantagespreviously discussed with regard to FIGS. 1-13 are equally applicablewith the use of a composite magnetic signal. As an example, monitoringmultiple in-ground lines using a composite magnetic signal may readilybe performed, with regard to the locating signal transmitted from theburied lines, by either assigning different transmission frequencies toeach line or by time sharing a single frequency.

Turning to FIG. 15, a first implementation of transmission section 318is generally indicated by the reference number 318 a. Transmissionsection 318 a includes a modulator 332 which is connected with anoscillator 334 producing a carrier frequency (not shown). Information tobe transmitted to the above ground location is received from CPU 316 andmodulated onto the carrier frequency by modulator 332. The modulatedcarrier frequency is then passed to an antenna 338 and transmitted as anRF signal 340 to the above ground location.

Referring to FIG. 16, a second signal implementation of transmissionsection 318 is generally indicated by the reference number 318 b.Transmission section 318 b utilizes an electrically insulated conductor342 which is fed down drill string 56 (only partially shown here, butalso shown in FIG. 10) during drilling. Conductor 342 may be used tosupply power to the instrumentation in the drill head and may also beused to send signals up and down the drill string. The drill stringitself serves as a second conductor. The arrangement of FIG. 16 providesbilateral communications using a line driver 344 and a signalconditioner 346 both of which are connected to a firstmodulator/demodulator 348. Line driver 344 receives appropriate datafrom CPU 316, amplifies the data and transfers the data tomodulator/demodulator 348. The latter modulates the data onto a carrier(not shown) and places the carrier onto conductor 342. At the drill rig(FIG. 10), the data is recovered by a second modulator/demodulator 350.Again, the manner of presentation or use of the data above ground may bedetermined in terms of convenience to the system operator or due tosecondary processing requirements. Second modulator/demodulator 350provides an input 352 for transfer of information down the drill stringto CPU 316. It is considered that design of arrangement 300 forincorporation into a drill head and programming may be performed bythose of ordinary skill in the art in view of this overall disclosure.

It should be understood that an arrangement for tracking the positionalrelationship between a boring tool and one or more buried lines and itsassociated method may be embodied in many other specific forms andproduced by other methods without departing from the spirit or scope ofthe present invention. Therefore, the present examples are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope of the appended claims.

1. In a system in which a boring tool, including a drill string having alength which is fed out in a way which moves the boring tool through theground in a region which includes at least one electrically conductivein-ground line, an apparatus for tracking a specific positionalrelationship between the boring tool and said line, said apparatuscomprising: a transmitter for generating a magnetic field from one ofeither said boring tool or at least from an underground length of saidline; a receiver carried by said boring tool if said magnetic field isgenerated from said line or in communication with said line if saidmagnetic field is generated from said boring tool for detecting arelative change in a particular characteristic of said magnetic field,which particular characteristic varies as a function of said specificpositional relationship between said boring tool and said line as theboring tool moves through the ground within said region; a drill stringmonitor for monitoring a value of at least one parameter relating to theway in which said drill string is fed out; and a processor that isconfigured for correlating the detected relative changes in saidcharacteristic with the value of said one parameter in a way whichindicates certain changes in the positional relationship between saidboring tool and said line which are indicative of the boring tool beingon a collision course with the line without the need to establish anactual distance between the boring tool and said line.
 2. The apparatusaccording to claim 1 wherein the processor is further configured forproviding a notification of said collision course.
 3. The apparatusaccording to claim 2 where said processor is configured to continuouslycorrelate data from the drill string monitor, regarding forward movementof the boring tool, with the relative change in the particularcharacteristic of the magnetic field.
 4. The apparatus of claim 3wherein said processor is configured for issuing a series ofnotifications based on the distance from the boring tool to the linechanging by a predetermined fraction.
 5. The apparatus of claim 4wherein the aforesaid line is a first line in said region which includesa plurality of lines and said processor is configured to separatelytrack the positional relationship for each line with the boring tool andto customize each of said notifications to identify the first line as aspecific one of the lines that is being approached.
 6. The apparatus ofclaim 1 wherein said particular characteristic is an intensity of saidmagnetic field and wherein said magnetic field intensity decreases in aparticular way as distance from the transmitter increases.
 7. Theapparatus of claim 1 wherein said magnetic field is generated from saidline and wherein said boring tool carries said receiver for detectingsaid particular characteristic of the magnetic field.
 8. The apparatusof claim 1 wherein said receiver includes an array of at least threeantennas which are positioned orthogonally with respect to one anotherso as to receive said magnetic field along three orthogonally disposedaxes.
 9. The apparatus of claim 1 wherein said particular characteristicis the intensity of said magnetic field and wherein said apparatusincludes an antenna array which produces a vector sum responsive to saidintensity at a particular location for identifying said positionalrelationship at that location.
 10. The apparatus of claim 9 wherein saidboring tool is oriented in a specific way with respect to said line andwherein said vector sum is indicative of said positional relationshipirrespective of the specific orientation of the boring tool with saidline.
 11. The apparatus of claim 1 wherein said magnetic field isgenerated from said boring tool and wherein said line is incommunication with said receiver for detecting said particularcharacteristic of the magnetic field.
 12. The apparatus of claim 11wherein said line is electrically connected directly with said receiver.13. The apparatus of claim 11 wherein said line is inductively coupledwith said receiver.
 14. The apparatus of claim 11 wherein said boringtool includes a drill head and wherein said transmitter is carriedwithin said drill head.
 15. The apparatus of claim 14 wherein saidtransmitter includes an array of antennas positioned in a predeterminedway with respect to one another.
 16. The apparatus of claim 15 whereinsaid transmitter further includes a transmission/control arrangementwhich drives said antenna array.
 17. The apparatus of claim 15 whereinsaid antenna array includes at least three antennas which are positionedorthogonally with respect to one another so as to permit transmissionfrom three orthogonally disposed axes.
 18. The apparatus of claim 17wherein said transmission/control arrangement includes individual driversections for driving each one of said three antennas at a differentfrequency.
 19. The apparatus of claim 18 wherein a first antenna isdriven at approximately 80 kHz, a second antenna is driven atapproximately 100 Hz below 80 kHz and a third antenna is driven atapproximately 100 Hz above 80 kHz.
 20. The apparatus according to claim17 wherein said transmission/control arrangement is configured forsimultaneously driving two of said three antennas 90 degrees out ofphase with respect to one another at a predetermined frequency and forswitching between alternate pairs of said three antennas at apredetermined switching rate such that the transmitter alternatelytransmits from said three orthogonally disposed axes.
 21. The apparatusof claim 20 wherein said predetermined frequency is approximately 80 kHzand said switching rate is approximately 1 kHz.